# Alkanes

> Edexcel International GCSE Chemistry · Edexcel IGCSE Chemistry 4CH1 (2017)
> Source: https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-alkanes/

This guide covers all core Edexcel IGCSE Chemistry 4CH1 content for alkanes, including their general formula, classification as saturated hydrocarbons, naming, structure drawing, and halogenation reactions under UV light.

**Prerequisites:** [Basic organic chemistry terminology (hydrocarbons, covalent bonding)](https://www.owlsprep.com/study/edexcel-igcse-chemistry-s4-intro-to-organic-chemistry/)

## Learning objectives

- Recall and apply the general formula for alkanes ($C_nH_{2n+2}$) for unbranched chains up to 5 carbon atoms
- Explain why alkanes are classified as saturated hydrocarbons
- Draw structural and displayed formulae, and name the first 5 unbranched alkanes (methane to pentane)
- Describe the mono-substitution reaction of alkanes with halogens under UV light, including balanced symbol equations

## Alkane General Formula & Saturated Hydrocarbon Classification

Alkanes are the simplest family of hydrocarbons, made exclusively of carbon and hydrogen atoms. They follow a predictable general formula that applies to all unbranched alkanes.

**Saturated Hydrocarbon** — A hydrocarbon containing only single covalent bonds between carbon atoms, with no double or triple C-C bonds present, so the maximum number of hydrogen atoms are bonded to each carbon.

*Example:* Methane ($CH_4$) is the simplest saturated hydrocarbon.

$$C_nH_{2n+2}$$

For $n$ carbon atoms, each alkane has $2n + 2$ hydrogen atoms. For example, if $n=3$ (propane), the formula is $C_3H_8$. This formula only applies to unbranched alkanes, which are the only ones you need to recall for this specification.

**Worked example:** Use the general alkane formula to find the molecular formula of an alkane with 5 carbon atoms.

1. Identify the value of $n$ from the question: $n = 5$
2. $$Substitute $n=5$ into the general formula $C_nH_{2n+2}$$$
3. $$Number of H atoms = $2 \times 5 + 2 = 12$$$
4. The molecular formula is $C_5H_{12}$ (pentane)

> **tip**
>
> You will not be given the general formula in your exam, so memorise $C_nH_{2n+2}$ to calculate molecular formulae quickly.

*Calculator:* allowed

## Naming Unbranched Alkanes (Up to 5 Carbon Atoms)

Unbranched alkanes have a straight chain of carbon atoms with no side groups. You are required to recall the names and formulae of the first 5 unbranched alkanes for your exam.

| Number of C atoms | Name | Molecular Formula |
| --- | --- | --- |
| 1 | Methane | $CH_4$ |
| 2 | Ethane | $C_2H_6$ |
| 3 | Propane | $C_3H_8$ |
| 4 | Butane | $C_4H_{10}$ |
| 5 | Pentane | $C_5H_{12}$ |

> **mnemonic**
>
> Use this memory hook to remember the first 4 prefixes: **M**any **E**lephants **P**lay **B**all → Methane, Ethane, Propane, Butane. Pentane follows the standard prefix for 5 (penta-).

**Check your understanding**

1. What is the name of the alkane with molecular formula $C_4H_{10}$?

   - Propane
   - Butane
   - Pentane
   - Ethane

   *Why:* $C_4H_{10}$ has 4 carbon atoms, so the prefix is 'but-' giving the name butane.

## Drawing Structural & Displayed Formulae of Alkanes

You may be asked to draw either structural formula (condensed, shows atom arrangement without all bonds) or displayed formula (shows every atom and every single bond) for alkanes up to 5 carbons.

**Displayed Formula** — A structural formula that shows all atoms and all covalent bonds between them in a molecule, with each bond represented as a single line.

*Example:* Displayed formula of methane shows a central C atom bonded to 4 separate H atoms with single lines for each C-H bond.

**Worked example:** Draw the displayed formula of propane ($C_3H_8$).

1. Draw three carbon atoms in a straight line, connected by single covalent bonds.
2. Add enough hydrogen atoms to each carbon so that every carbon has 4 bonds total (carbon always forms 4 covalent bonds).
3. The two end carbons bond to 3 hydrogen atoms each, and the middle carbon bonds to 2 hydrogen atoms, giving a total of 8 hydrogen atoms.

> **warning**
>
> If a question asks for displayed formula, you must draw every single C-C and C-H bond. Omitting bonds will cost you marks, even if the overall structure is correct.

**Check your understanding**

1. Which of the following is a requirement for displayed formulae?

   - Only show C-C bonds
   - Show all C-C and C-H bonds
   - Show only the carbon chain
   - Use condensed groups like $CH_3$

   *Why:* Displayed formulae explicitly show every covalent bond in the molecule.

## Alkane Halogenation Reactions

Alkanes are generally unreactive with most common reagents, but they react with halogens (e.g. chlorine, bromine) in the presence of ultraviolet (UV) light in a substitution reaction.

**Mono-substitution Reaction** — A reaction where one hydrogen atom in the alkane molecule is replaced by a single halogen atom, forming two products: a halogenoalkane and a hydrogen halide.

The only condition you need to recall for this reaction is UV light (e.g. sunlight). Reaction mechanisms (such as free radical steps) are explicitly excluded from the specification, so you do not need to learn these.

$$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$$

In this reaction, one hydrogen from methane is replaced by one chlorine atom, making chloromethane (the halogenoalkane) and hydrogen chloride (the hydrogen halide). This is mono-substitution, so only one H is replaced.

**Worked example:** Write the balanced symbol equation for the mono-substitution reaction between ethane ($C_2H_6$) and bromine ($Br_2$) under UV light.

1. Identify the reactants: ethane ($C_2H_6$) and bromine ($Br_2$), with UV as the condition.
2. One H atom from ethane is replaced by one Br atom, forming bromoethane ($C_2H_5Br$) and hydrogen bromide ($HBr$).
3. $$C_2H_6 + Br_2 \xrightarrow{UV} C_2H_5Br + HBr$$
4. Check the equation is balanced: 2 C, 6 H, 2 Br on both sides, so it is correct.

> **Exam tip:** Always include the UV light condition above the reaction arrow to gain full marks for halogenation questions.

## Common pitfalls

- **Wrong:** Using the general formula $C_nH_{2n}$ for alkanes
  - Why it fails: That formula applies to alkenes, not alkanes, so you will calculate incorrect molecular formulae
  - Correct: Memorise the alkane general formula $C_nH_{2n+2}$, and double check H counts for each alkane
- **Wrong:** Drawing displayed formula without showing all C-H bonds
  - Why it fails: Displayed formula requires every covalent bond to be visible, so missing bonds lose marks
  - Correct: Count the bonds around each carbon to ensure it has 4 bonds total, drawing every C-H and C-C bond explicitly
- **Wrong:** Naming an alkane with 4 carbon atoms as pentane
  - Why it fails: Prefixes correspond to number of carbons: but- is 4, pent- is 5
  - Correct: Memorise the prefixes for 1-5 carbon atoms: meth(1), eth(2), prop(3), but(4), pent(5)
- **Wrong:** Writing the halogenation reaction without UV as a condition
  - Why it fails: Alkanes do not react with halogens at room temperature without UV light, so the condition is required for full marks
  - Correct: Always write UV above the reaction arrow for alkane halogenation equations
- **Wrong:** Drawing di-substituted products for alkane halogenation reactions
  - Why it fails: The specification only requires knowledge of mono-substitution (one H replaced) for this topic
  - Correct: Only write equations where one H atom is replaced by one halogen atom, forming one halogenoalkane and one hydrogen halide

## Cheatsheet

| Concept | Key Details to Recall |
| --- | --- |
| General Formula | $C_nH_{2n+2}$ for all unbranched alkanes |
| Saturated Hydrocarbon Definition | Only single C-C bonds, no double/triple bonds |
| First 5 Alkanes | Methane (1C), Ethane (2C), Propane (3C), Butane (4C), Pentane (5C) |
| Displayed Formula Requirement | Show all C-C and C-H covalent bonds explicitly |
| Halogenation Reaction | Alkane + Halogen --UV--> Halogenoalkane + Hydrogen halide (mono-substitution only) |

## What's next

Now that you have mastered the core properties of alkanes, you can move on to related organic chemistry topics in the Edexcel IGCSE Chemistry 4CH1 specification. Alkanes are the foundational family of hydrocarbons, so understanding their structure and reactivity will help you grasp the key differences between saturated and unsaturated hydrocarbons when you study alkenes next. You will also use your knowledge of alkane properties when learning about crude oil fractionation, cracking, and the use of alkanes as fuels in earlier organic chemistry subtopics. Make sure you practice drawing displayed formulae and writing halogenation equations regularly, as these are common 1-3 mark questions in both Paper 1 and Paper 2 exams.

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